Sub-seasonal rainfall fluctuations, characterized as active and weak spells during the Indian monsoon season (June to September) is an important component of the variability of the Indian monsoon rainfall. A new criteria for identification of active and break events (Rajeevan et al. 2008) of the Indian summer monsoon have been recommended on the basis of the high resolution daily gridded rainfall data set over India (1951-2007) (Rajeevan et al. 2006). Based on this criteria, the active and break events were identified from the average rainfall data over a critical area, called the core monsoon zone (shown in the Figure below as a box) within which the monsoon trough/ Continental Tropical Convergence Zone (CTCZ) normally fluctuates. The active and break spells for the period 1901-2015 (Pai et al. 2015) were derived using IMDs new high resolution (0.25 x 0.25 degree) daily gridded rainfall data set (Pai et al. 2014). Active and break events are defined as periods in which the normalized anomaly of the rainfall over the monsoon zone exceeds 1 or is less than -1.0 respectively, provided the criterion is satisfied for at least three consecutive days.

The time series of standardized rainfall anomaly over the monsoon zone from 1 June 2023 using the IMDs 0.25 x 0.25 degree resolution daily gridded rainfall data is given below. This time series will help you to monitor the active and break events in this monsoon season.

References:

• Rajeevan, M., Bhate, J., Kale, J.D. and B. Lal, 2006, High resolution daily gridded rainfall data for the Indian region: Analysis of break and active monsoon spells, Current Science, 91, 3, 296 -306.
• Rajeevan, M, Sulochana Gadgil, and Jyoti Bhate, 2008, Active and Break Spells of the Indian Summer Monsoon, NCC Research Report No. 7 /2008, 45 pp.
• Pai. D. S., Latha Sridhar, M Rajeevan, O. P. Sreejith, N. S. Satbhai, and B. Mukhopadhyay, 2014, Development of a new high spatial resolution (0.25×0.25 degree) long period (1901-2010) daily gridded rainfall data set over India and its comparison with existing data sets over the region, Mausam, p1-18.
• Pai, D.S., Latha Sridhar & Ramesh Kumar, M.R., 2015, Active and Break events of Indian Summer Monsoon during 1901-2014, Clim.Dyn. Doi 10.1007/S00382-015-2813-9.